Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.

pkgs/tools/networking/axel/default.nix

@@ -8,6 +8,6 @@ stdenv.mkDerivation {
   };
 
   meta = {
-    description = "A console downloading program. Has some features for parallel connections for faster downloading.";
+    description = "Console downloading program with some features for parallel connections for faster downloading";
   };
 }